1. Field of the Invention
The present invention relates generally to electronic control units and more particularly, to an electronic control unit for use in a vibratory feeder system.
2. Prior Art
In the automatic assembly of complex mechanisms, the feeding of parts to assembly devices is usually carried out by means of vibratory feeders. Such feeders are also used for dispensing small parts of quantities of powders or the like to packaging or dispensing machines.
Vibratory feeders are generally either (i) linear moving feed mechanisms or (ii) bowl-shape feeders which vibrate about an axis along the centerline of a feeder bowl which carries the product to be dispensed. Feeder bowls typically include an inclined ramp located along the inside circumference of the bowl which spirals from the bottom portion of the bowl to the bowl top. An electromagnetic drive device is provided for vibrating the bowl about the bowl centerline thereby causing the product at the bottom of the bowl to slowly travel up the inclined ramp to a guiding chute which guides the product from the periphery of the bowl to one or more assembly stations.
The bowl feeder is typically resiliently supported by a number of springs, the lower ends of which are attached to a heavy base which is in turn supported by cushioned feet. Flat springs whose longitudinal axis lies approximately 15.degree. C. off the vertical are commonly used. The feed bowl is secured to the upper ends of the springs, with the electromagnetic drive device being positioned between and coupled to the rigid base and the resiliently mounted bowl.
In common practice, the electromagnetic drive device is driven directly by a 115 volt A.C. line with the line frequency being either 50 Hz or 60 Hz. The drive device will accordingly deliver drive pulses to the bowl at twice the line frequency, i.e., either 100 Hz or 120 Hz. If the frequency of the driving force is largely different from the resonant frequency of the mechanical system being driven, the magnitude of displacement will be minimal. Accordingly, it is desirable to drive the bowl at the resonant frequency of the mechanical system in order to maximize the movement of the bowl for a given driving force. The current practice has been to manually tune the mass-spring relationship of the feeder system so that the mechanical resonant frequency matches the drive frequency.
The prior art feed control systems possess several shortcomings. By way of example, any change in the mass of the mechanical system causes a shift in the resonant frequency of the feeder system. Mass changes occur, of course, as the product is delivered to the guide chute and when the feed-bowl is refilled. Inasmuch as the drive frequency no longer coincides with the shifted system resonant frequency, the amplitude of bowl vibration will be reduced thereby causing an undesireable decrease in feed rate of the product.
One method currently used to overcome the limitations in the prior art systems is to carefully manually adjust the mechanical system so that load changes cause a minimal shift from the natural frequency of the system. This technique requires considerable skill, is time-consuming and is not always effective.
A second technique currently used is to insert a variable transformer between the electromagnetic drive and the line source. As the resonant frequency of the feed system shifts, an operator changes the drive voltage by manually adjusting the variable transformer so as to maintain a constant product feed rate. This technique is disadvantageous in that an operator must continually monitor the feed rate and make the necessary corrections in drive voltage not only to compensate for changes in the bowl mass, but also to compensate for changes in line voltage and temperature. Moreover, any significant mismatch between the system resonant frequency and the drive frequency necessitates a substantial increase in drive power in order to maintain a constant feed rate. The increased power requirement causes a wasteful expenditure of energy and may result in overheating of the system
Another technique which is sometimes used to overcome the deficiencies in the prior art vibrator control systems is to power the electromagnetic driver from a power source which has provisions for manual adjustment of the drive frequency. Again, this technique requires the constant attention of an operator. Furthermore, the output voltage of available variable frequency power sources typically varies with the input line voltage, thus necessitating further adjustments by the operator.
Some prior art systems utilize a feedback device mounted on the feed bowl which is used in conjunction with appropriate control circuitry for controlling the drive rate of the system in order to maintain a constant feed rate. Such systems do not have variable frequency control. Accordingly, large shifts in resonance necessitates large increases in drive power which result in energy wastage and possible overheating.
Prior art vibrator control and related systems are disclosed in U.S. Pat. No. 3,447,051 issued to Attwood et al,; U.S. Pat. No 3,716,130 issued to Morris; U.S. Pat. No. 4,002,270 issued to Reiner and U.S. Pat. No. 4,038,558 issued to Woolfson et al. While such systems overcome some of the aforementioned deficiencies, serious limitations remain.
The present invention overcomes the limitations inherent in prior art vibrator feeder control systems by providing a control unit which automatically adjusts the frequency and/or the magnitude of the electromagnetic device drive voltage to compensate for changes in feeder loading and the like. The control unit of the present invention is reliable, does not require the constant attention of an operator, and is not affected by variations in temperature, line voltage or line frequency.